1. Field of the Invention
The present invention relates to a holographic recording medium, and a holographic recording/reproducing apparatus using the same. More particularly, the invention relates to a two-color holographic recording medium which not only realizes high recording sensitivity without reducing treatment but also can reduce optical damage produced by the irradiation of gating light, and a two-color holographic recording/reproducing system using the same.
2. Description of the Related Art
Recently, accompanying to the increase in information capacity to be recorded in information recording medium, the requirement for the recording medium provided with potential of huge capacity and rapid readout (fast data transfer) is accelerating. One of the candidates for such a medium is a holographic memory medium using ferroelectric crystal of, for example, lithium niobate (LiNbO3), a lithium tantalate (LiTaO3), or the like.
Conventionally, a holographic memory system is an optical data storage system in which information is three-dimensionally recorded by use of laser, in a manner of volumetric multiplexing based on principles of volume holography. The photorefractive effect utilized in the medium is a phenomenon in which electric charges generated by photoexcitation caused by irradiation with light move within the crystal to form a space charge distribution, and the space charge distribution changes the refractive indices of the crystal via a linear electro-optical effect, i.e. the Pockels effect in a manner corresponding to the space charge distribution.
There are two recording methods employed by the above holographic memory: one-color holography and two-color holography. The one-color holography suffers from a problem, or what is called “reproduction deterioration” that reproduction light progressively erases a recorded hologram when the reproduction light reads out signals from the hologram. Two-color holography is a recording method, which solves the issue of the erasure during readout in one-color holography.
FIG. 9 illustrates the recording mechanism of two-color holography. Energy band structure 900 of the holographic recording medium which adopts two-color holography consists of the valence band (VB), the conduction band (CB), and three energy levels, i.e. a level A, a level B, a level C between the valence band (VB) and the conduction band (CB). The energy level A (light absorption center or bipolaron) exists at the deeper energy position than the energy level B (metastable intermediate level or small polaron) when measured from the bottom of the conduction band. The energy level C (trap level or storage center) exists at the deeper energy position than the energy level B. Next, the mechanism of recording information into the holographic medium using two-color holography is explained.
Holographic recording medium is irradiated with a gating light (at a wavelength λ1) to create carriers responsible for photorefractive effect. In the portion irradiated with the gating light, the carriers are excited from the level A to the conduction band (CB), and temporarily trapped at the intermediate level B. A carrier lifetime at the level B is τ1.
The medium is irradiated with recording lights in order to record information. Recording light consists of a reference light (at a wavelength λ2) and a signal light carrying information to be recorded (at a wavelength λ2). Wavelength λ1 and wavelength λ2 have to obey the relationship of λ1<λ2. The carriers at the intermediate level B are excited into the conduction band (CB) by irradiating recording light according to the spatial intensity profile of interference fringes formed by a reference light and a signal light, and finally accumulated at the trap level (storage center)C in the form of a concentration distribution of the carriers corresponding to the interference fringes, to complete the recording operation. A carrier lifetime at the level C is τ2.
As specific means for two-color holography using lithium niobate, there has been proposed a method using single-crystal lithium niobate (LN) subjected to reduction treatment. One example of this material is a crystal of reduced lithium niobate doped with praseodymium (Pr) (H. Guenther, G. Wittmann, and R. M. Macfarlene (IBM), R. R. Neurgaonkar (Rockwell); “Intensity dependence and white-light gating of two-color photorefractive gratings in LiNbO3”, Opt. Lett. vol. 22, pp. 1305-1307 (1997)). Another example is reduced lithium niobate crystal undoped or doped with Fe or Mn (L. Hesselink, S. S. Orlov, A. Liu, A. Akella, D. Lande, and R R. Neurgaonkar: “photorefractive Materials for Nonvolatile Volume Holographic Data Storage”, Science Vol. 282 (November 6), pp. 1089-1094 (1998)).
However the two-color holographic recording materials mentioned above are required to be subjected to reduction treatment in order to obtain enough sensitivity for the use in holographic recording medium. More specifically, the materials can be hardly used as recording materials in an as-grown state or in a state subjected to heat treatment in the air since they have a low recording sensitivity in such a state (for example, sensitivity is in the order of 10−4 cm/J or lower). Reduction is inevitable process to establish energy level A occupied by enough electrons in the materials.
Further, if reduction treatment is excessively carried out, the dark conductivity of the crystal is increased to shorten storage time, causing a problem in the practical use thereof. Further, it is difficult to control the two-color holographic memory system since the holographic properties (sensitivity etc.) of the materials are largely varied depending on the degree or condition of the reduction treatment (such as temperature, atmosphere, time etc.).
For example, in the two-color holographic recording materials disclosed in the latter reference mentioned above, a gating light at a wavelength from 458 nm to 488 nm is used. When the gating light at the relatively longer wavelength is employed, some kind of light scattering (optical damage) called beam fanning is liable to be caused by the photorefractive effect. When beam fanning occurs, an image reproduced from information written in the holographic memory is deformed, and data is degraded, so that the quality of the hologram is largely reduced.
Besides, in the two-color holographic recording material disclosed in the latter reference mentioned above, optimization of Mn content has not been established.
Consequently, the object of this invention is to provide a holographic recording medium in which a high recording sensitivity can be obtained without reduction treatment and optical damage under the irradiation is reduced, and to provide a holographic recording/reproducing system using the same.
In addition, it is to be noticed that the term “optical damage” used herein is optical scattering caused by the exposure of gating light and does not mean photorefractive effect for creating the hologram for the information storage.